Conventionally, a vehicular headlight device which detects a preceding vehicle driving in front of an occupant's vehicle by using a camera and controls a light distribution of headlights depending on a position of the preceding vehicle is known (see Patent Document 1 for example).
This vehicular headlight device determines whether a preceding vehicle has moved into a predetermined partial region in an image captured by the camera. The vehicular headlight device sets margins adjacent to either sides of the partial region in consideration of an installation error and a detection error of the camera or component tolerance and an installation error of a headlight unit. Then, the vehicular headlight device determines that the preceding vehicle is within the partial region if the vehicular headlight device detects the preceding vehicle within an enlarged region including the margins. Then, the vehicular headlight device switches off light emitting elements which irradiate the partial region.
Alternatively, the vehicular headlight device determines that the preceding vehicle is going to be within the partial region before the preceding vehicle actually goes into the partial region or the enlarged region including the margins if a horizontal moving velocity of the preceding vehicle which horizontally moves toward the partial region is greater than a predetermined value. Then, the vehicular headlight device switches off light emitting elements which irradiate the partial region.
In this way, the vehicular headlight device switches off related light emitting elements without a delay when an occupant's vehicle makes a turn, even if the preceding vehicle goes into the partial region at an apparent high speed, so that it can prevent a driver of the preceding vehicle from being dazzled.